Abstract

Have you ever noticed that the salt you are using says it is "iodized"?
Iodine is an important micronutrient, which means we need it in small quantities to be healthy. Because iodine is rare in many people's normal diets, it is added to table salt. Then when people salt their food, they are also adding this important micronutrient. In this food science project, you will use some kitchen-friendly chemistry to investigate which types of salt have iodine added (in the form of iodide) and which do not. Get ready to shake out some salt!

Objective

Test different types of salt to see if they contain iodine, a micronutrient.

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Credits

Teisha Rowland, PhD, Science Buddies

This science project idea is based on the following classroom activity by Stephen W. Wright of Pfizer Global Research and Development:

Introduction

Did you know that the number one thing people can do, world wide, to prevent mental retardation is to cook and season food with iodized salt? Here is why. Nutrients are chemicals that people need for their bodies to run normally and be healthy. Micronutrients, like iodine, are types of nutrients that people need in small amounts. Iodine is specifically very important for a person's thyroid to function normally. (The thyroid is a gland in the neck that makes key hormones.) If a person does not eat enough iodine in their diet, they can become iodine deficient. Iodine deficiency can cause different medical problems (usually due to hypothyroidism, or having a thyroid that does not make enough hormones). This includes goiter, which is a visible swelling of the thyroid, serious birth defects like cretinism, and brain damage. In fact, iodine deficiency is the most common cause of preventable mental retardation.

Iodine deficiency is completely preventable by making sure people get enough iodine in their diets, which is why iodine (in the form of iodide) is added to table salt. Iodine is rare in many people's normal diets, but if they use iodized salt, then when they salt their food they are also getting a small dose of this important micronutrient. Since the 1980s, efforts have been made to have universal salt iodization. This has been an affordable and effective way to combat iodine deficiency around the world. However, not all types of salt contain iodine.

In this food science project, you will use some kitchen-friendly chemistry to investigate which types of salt have iodine added to them. Which do you think will have iodine, and which do you think will not? To do this test you will use laundry starch, vinegar, and hydrogen peroxide. Iodine undergoes a chemical reaction with starch to make a blue-purple-colored chemical, as shown in Figure 1, below. This is how you can visibly see if there is iodine. Vinegar and hydrogen peroxide are added to the salt solution to help the iodine reaction with starch take place. Specifically, vinegar is an acid, and so when you add it you are changing the pH of the solution so that the reaction can take place. (You do not need to understand acids and pH to do this science project, but if you would like to find out more about them, you can check out the Science Buddies' resource
Acids, Bases, & the pH Scale).
Lastly, since salt has iodine in the form of iodide, you will add hydrogen peroxide to turn the iodide into iodine, which the starch will react with.

Figure 1. Within the correct pH range, starch undergoes a chemical reaction with iodine to turn it a blue-purple color, as shown here.

Terms and Concepts

Nutrients

Micronutrients

Iodine

Iodine deficiency

Goiter

Iodized salt

Chemical reaction

Questions

What medical complications can happen if someone has iodine deficiency?

Assuming iodized table salt contains 0.006% iodide by mass, how much iodized salt would a person need to consume each day in order to get the recommended 150 micrograms of iodide?

How can you tell if iodine has chemically reacted with starch?

Which types of salt do you think will have iodine (in the form of iodide), and which do you think will not? Why? Hint: Look at the labeling on the packaging for the different types of salt.

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Materials and Equipment

Permanent marker

Disposable plastic cups, 10-oz. or larger (at least 6). You will need one for the positive control iodine solution and one for each type of salt you want to test.

Tip: In order to see the iodine-starch reaction, the cups should be clear plastic or colored plastic with white interior.

100 mL graduated cylinder. Graduated cylinders are available online through suppliers such as
Carolina Biological. Alternatively you could use measuring cups.

Distilled water (at least 1.5 L)

Note: Using tap water can produce erratic results.

10 mL graduated cylinder. Graduated cylinders are available online through suppliers such as
Carolina Biological. Alternatively you could use measuring spoons.

Laundry starch solution, also called liquid starch (at least 15 mL). This can be found in the laundry detergent section of the grocery store.

If you cannot find laundry starch solution, you can make a starch solution by mixing 5 g (1 to 2 tsp.) cornstarch with 30 mL (2 Tbsp.) cold water, stirring this solution into 500 mL (2 C) of vigorously boiling water, and then allowing it all to cool. Be sure to have an adult help you if you use a stovetop. Alternatively, 1 C of starch-based biodegradable packing peanuts could be dissolved in 500 mL water to make a suitable starch solution.

Iodine antiseptic solution (5 drops)

Use either an iodine tincture or povidone-iodine solution, found in the first aid section of grocery stores and drugstores.

Medicine dropper. This is in the first aid section of grocery stores and drugstores.

Disposable plastic spoons (at least 6). You will need one for the positive control and one for each type of salt you want to test.

At least five different types of salt (4 Tbsp, or about 80 g, of each). For example:

Plain (non-iodized) table salt

Iodized table salt

Pickling salt

Rock salt, such as is used for making ice cream. Be sure to filter any dirt from the solution before testing.

Kosher salt

"Lite" salt

Sea salt

Note: Some nations permit the use of potassium iodate (KIO3) as an iodine supplement in iodized salt. The procedure used in this science project will not detect iodine added as potassium iodate. The U.S. Food and Drug Administration (FDA) has not approved the use of potassium iodate as a food additive in the U.S.

3% hydrogen peroxide solution (at least 75 mL)

White vinegar (at least 75 mL)

Optional: Camera

Lab notebook

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Add five drops of iodine antiseptic solution and stir well with a clean, disposable plastic spoon. What do you observe? You should see the iodine turn a blue-purple color as it interacts with the starch solution, as shown in Figure 1, below.

Figure 2. You should see the iodine turn a blue-purple color as it interacts with the starch solution.

Testing Various Types of Salt for the Presence of Iodide

In your lab notebook, you could create a data table to record your results in. You will want to have a row for each type of salt you test (and the positive control iodine solution) and a column where you can record the color of each solution at the end of their test.

Pick one of the types of salt to test. Using the permanent marker, label a clean, plastic cup with the type of salt that you are testing. Place 4 tablespoons (Tbsp.) (about 80 grams [g]) of the salt into the labeled cup, as shown in Figure 3, below.

Figure 3. For each type of salt you test, you will need to measure out 4 Tbsp. into a clean, disposable plastic cup.

Add 240 mL (1 C.) of distilled water and stir well for about a minute with a clean, disposable plastic spoon. Not all of the salt will dissolve, but any iodide present in the salt will dissolve.

Add 15 mL (1 Tbsp.) of white vinegar, as shown in Figure 4, below.

Add 15 mL of 3% hydrogen peroxide.

Figure 4. After mixing distilled water with the salt, add 1 Tbsp. of the vinegar and 1 Tbsp. of the hydrogen peroxide to help the chemical reaction between iodine and starch take place.

Add 2.5 mL (1/2 tsp.) of the starch solution.

Stir the salt solution well with the disposable plastic spoon (using the same spoon you already used with this solution), and then let it stand for a few minutes. Does the solution become a blue-purple color, indicating that it has iodine? Record your results in your lab notebook.

Repeat steps 2-7 using different types of salt. Be sure to use a different, clean disposable cup and spoon for each different type of salt you test so you do not contaminate your results. Also record all of your results in your lab notebook and, if possible, take pictures.

Analyze your results. Which types of salt have detectable amounts of iodine? Are you surprised by any of your results?

Some types of salt say whether or not they contain iodine (in the form of iodide) on their packaging. Take a look at the containers for the different types of salt that you tested. Do your tests confirm, or disprove, the containers' labeling?

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Variations

In this science project, you added vinegar because it is an acid that helps the chemical reaction take place between the starch and the iodine. You could investigate the effect of pH on this chemical reaction by repeating the experiment but not using vinegar. Do you still get a blue-purple color when mixing an iodized salt solution with the hydrogen peroxide and starch, or do you get no color change at all? Or does the chemical reaction just take more time to happen? Hint: You may need to be patient to see the results of this experiment! To find out more about acids and pH, check out the Science Buddies' resource
Acids, Bases, & the pH Scale.

The rate of the chemical reaction you explore in this science project can be affected by chemicals known as reducing agents. Corn syrup (which is primarily a solution of sugars with water) is a reducing agent you could experiment with. Try adding 1-2 tsp. of corn syrup to the iodized salt solution. How does adding the corn syrup affect how quickly the iodine changes color when it is mixed with the starch? You may need to do some research on the chemical reaction used in this science project, and reducing agents, to explain your results. You could start by looking at the Wright, S. W. (2007) resource listed in the
Bibliography.

Temperature often affects chemical reactions. You could try this experiment again, but this time pick one salt solution that changed color and test it at different temperatures by using distilled water that has been cooled or heated to different temperatures. (You can measure and record the temperatures using a thermometer.) How does changing the temperature of the solution change how the color-changing reaction takes place?

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